This invention relates to an optical scanning device provided with a rotary polygon mirror, and to an image forming apparatus incorporating such an optical scanning device.
In using an optical scanning device having a rotary polygon mirror, dust in air collides against the reflective faces of the rotary polygon mirror and sticks to the reflective faces to become dirt as service time increases. The dirt deteriorates a reflectivity at the reflective faces, which decreases optical power at an optical beam spot on a scanned face. In this phenomenon, the dirt formed on each reflective face is not uniform, so that an optical power distribution on the scanned face in a primary scanning direction also does not become uniform. When an electrophotography laser printer equipped with such an optical scanning device is used continuously, the non-uniformity of density or color of images, caused by the non-uniformity of optical power distribution, will result.
Conventionally, the rotary polygon mirror is enclosed in an airtight environment to prevent such non-uniformity of optical power distribution due to the dirt on the reflective faces of the rotary polygon mirror. This method, however, is not effective enough to completely prevent the dirtying of the reflective faces, thus the non-uniformity of density or color of images cannot be avoided when the optical scanning device is operated for a longer period.